avast doesn't scan it as a matter of course, it is an on-access scanner, so something must access the file/s to initiate a scan of the accessed file/s.
This obviously doesn't scan all files on the CD/DVD (could take a very long time) or usb flash drives, only those files accessed or run depending on your standard shield sensitivity and settings. When this is set to High it will scan more files but these too would first have to have had something try to run or access them first.
Using windows explorer you could right click on the drive letter and select Scan selected areas for viruses, this would use the ashQuick.exe scan which is the most thorough of the scans.